1. Field of the Invention
The present invention relates to a hybrid direction identifying apparatus and method that rapidly and accurately detects a direction of a radio signal source based on a direction detecting scheme by comparing an amplitude or a signal strength, a virtual Doppler scheme, a radio phase interferometry scheme, that is, a phase difference comparison scheme, and the like.
2. Description of the Related Art
A radio direction identifying method may use an angle of arrival (AoA) scheme, and the AoA scheme may include an amplitude or signal strength comparison scheme, a Doppler scheme, a phase comparison scheme, and the like. An accuracy of the amplitude or signal strength comparison scheme may be enhanced as a size of an antenna increases, in particular, when a 3 decibel (dB) beam width deteriorates, since a direction is set to be a point at which a signal strength is at maximum in a state in which a directional antenna is rotated or fixed. In the Doppler scheme, a higher precision may be achieved with an increase of a rotation per minute (RPM) and a rotation radius, since a circular arrayed omni-directional antenna is required to be rotated electrically to make a virtual Doppler. The phase comparison scheme may identify a direction of a signal by measuring a phase difference of the signal arriving to at least two fixed omni-directional antennas as shown in FIG. 1 and Equation 1.
                    Δϕ        =                              -                                          2                ⁢                                                                  ⁢                π                ⁢                                                                  ⁢                d                            λ                                ⁢                      sin            ⁡                          (              φ              )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, Δφ denotes a phase difference, σ denotes an AoA, and d denotes a distance between two antennas.
The amplitude or signal strength comparison scheme of the AoA scheme may have a limit in terms of a size of an antenna since a highly directional antenna is required for a high degree of precision. The virtual Doppler scheme may be complicated and costly since thousands to tens of thousands of RPM electrical rotations would be required. A phase difference comparison scheme of two fixed antennas may require at least five circular array antennas to detect all directions in a range of 0 degrees to 360 degrees, and achieve a higher precision as a number of array antennas increases. Accordingly, as the number of array antennas increases, a number of radio frequency (RF) elements may become greater and signal processing may become increasingly complicated.
Achieving a certain degree of precision using the amplitude or signal strength comparison scheme, the Doppler scheme, and the phase difference comparison scheme of two fixed antennas may face restrictions due to a high cost and an implementation of the schemes. The amplitude or signal strength comparison scheme of the AoA scheme may require an antenna having a relatively great aperture, and the phase difference comparison scheme of two fixed antennas in which a virtual Doppler and at least five array antennas are present may be complicated and require a high cost. The virtual Doppler scheme and multiple array antennas scheme, that is, at least five, may be complicated and require a high cost since at least five antennas are required to be arrayed and configuration of a great number of components is required to obtain a precision of one to three degrees.
The amplitude or signal strength comparison scheme may enable an AoA with a low precision, although fast, without an ambiguity. In the phase difference comparison scheme of two fixed antennas, when a distance d is λ/2 in Equation 1, an ambiguity may be 180 degrees, and φ being an AoA may have an error as shown in Equation 2 and FIG. 2 due to a mutual coupling between antennas.
                              Δ          ⁢                                          ⁢          φ                ≈                  -                      λΔσ                          2              ⁢              π              ⁢                                                          ⁢              d              ⁢                                                          ⁢                              cos                ⁡                                  (                  φ                  )                                                                                        [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, Δσ denotes a phase measurement error, and φ denotes an AoA. Accordingly, an error Δσ of an AoA may increase as an incident angle φ, the AoA, of a radio wave increases when an array antenna is fixed. However, when the incident angle φ is zero, that is, vertical to an array antenna axis, the error Δσ may be at a minimum. The phase difference comparison scheme of two fixed antennas may use a principle in which an error becomes the minimum when a signal enters two antennas simultaneously. More particularly, the phase difference comparison scheme of two fixed antennas may identify a direction at which a phase difference is zero by rotating an antenna.